Israel also held up shipments of European Union-funded fuel to the territory's sole power plant. Palestinian officials said the facility would be shut down later in the day.
"The border crossings will not be opened today, despite our plans, due to the continued rocket fire and security threats at the crossings," said Defence Ministry spokesman Peter Lerner.
Israel had initially said it would allow the delivery of fuel and some 30 truckloads of food and other humanitarian supplies into the enclave, where a flare-up in cross-border fighting threatens a 5-month-old truce.
"Our warehouses are empty and our food distribution to 750,000 people will be suspended if we cannot get food in," said Christopher Gunness, spokesman for the United Nations Relief and Works Agency (UNRWA).
Some 1.5 million people live in the Gaza Strip. Residents said on Thursday that food remained available in the Gaza Strip but certain items were in short supply.
Gunness said Israeli officials were non-committal about allowing in supplies on Friday. The crossings are generally closed mid-day Friday through Sunday.
The agency said distribution to Gazans would have to stop on Thursday night because supplies would have run out.
UNRWA head Karen AbuZayd told Reuters in an interview in Brussels she was worried Israel was narrowing the criteria for humanitarian aid and that certain items, including some school supplies, would be excluded from future shipments.
Palestinian militants fired several rockets at southern Israel earlier on Thursday, causing no casualties, the Israeli army said, a day after soldiers killed four Hamas gunmen during a raid into the Gaza Strip.
Israel says it remains committed to the ceasefire, which was brokered by Egypt.
"Without a doubt, it is faltering, but it isn't over," Israeli Deputy Defence Minister Matan Vilnai told Israel Radio.
Israel has not allow the U.N. and other agencies to bring supplies into the Gaza Strip since Nov. 4, when its troops raided the territory to destroy what the army described as a tunnel built by militants.
Six Hamas gunmen were killed. Militants responded to the incursion with rocket salvoes.
The ceasefire, which began in June, calls on both sides to stop cross-border violence and on Israel to ease the Gaza blockade it tightened after Hamas Islamists seized the territory more than a year ago.
(Reporting by Adam Entous in Jerusalem, Nidal al-Mughrabi in Cairo and Ingrid Melander in Brussels; Editing by Louise Ireland and Giles Elgood)
- Reuters - Thomson Reuters Foundation
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